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Poll: Does your 915 w/ LSD make a binding/grinding noise or sensation when turning sharp?
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Does your 915 w/ LSD make a binding/grinding noise or sensation when turning sharp?

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Those with a LSD, I got a ?

My 1982 911 SC has a ZF limited slip differential. When starting from a stop and making a turn in either direction while accelerating I feel a very strong binding/grinding/crunching/vibrating sensation until I straighten out. I never turn hard enough above 1st gear to reproduce the situation. Is this normal? Someone once told me "Well at least you know its working"....but there's no way this is what its supposed to feel like, is it?

Old 05-20-2014, 07:15 PM
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My '84 does not do this. I make a full lock turn out of my parking spot at work every day I drive the car. Never have noticed anything then.
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Old 05-20-2014, 08:11 PM
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Nope - mine does not do that -
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Old 05-20-2014, 08:23 PM
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Someone restacked your LSD at some point. It is more aggressive than stock. Either that or it has oil in there that is too sticky for a street car. What do you run?
Old 05-20-2014, 10:13 PM
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You may have the wrong gear oil in your transmission (as Matt said).
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Old 05-20-2014, 11:38 PM
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The ZF LSD in my former 930 would also groan a bit on full lock turns from a standing start. 40%, with 50K miles.
Old 05-20-2014, 11:53 PM
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My 930 had this symptom. Changed to Kendall 80W90 LS which totally remedied the noise. Do a search on gear oil - there are many narratives describing why you are hearing the grinding, and great recommendations on preferred gear oil, and what not to use.
Old 05-21-2014, 03:02 AM
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Ok I've added friction modifier to the fluid that's in there. The fluid looks fresh but I'm not sure what weight/brand it is. Again this is not just a noise it's a full on sensation of metal on metal. I really don't like the feeling. It feels like something is being damaged.


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Old 05-21-2014, 04:47 AM
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Yes-- my 1981SC does exactly this. Will also do it around a low speed hairpin in second gear. The PO changed the gear oil to Mobilube before I bought the car, I'll be trying the Kendall that others recommended to see if that alleviates the noise. My car also has the factory ZF LSD, 159K miles.
Old 05-21-2014, 04:56 AM
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My '83 915 gearbox, factory LSD with kendall oil does not do this. It is completely quiet.
Old 05-21-2014, 05:57 AM
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The problem with adding a friction modifier is that the same oil is also used for the transmission. You have a 915 I believe and I don't think the Porsche synchros will work properly with a slippery additive. I would drain it all out and start with fresh, correct fluid.
Old 05-21-2014, 06:03 AM
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Ok I'll order some Kendall tranny sauce and will report back. This is one of those moments where I'll be shocked if by changing the fluid this metal on metal thrashing sensation is fixed. Guess it's the cheapest possible fix so it's worth a shot.


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Old 05-21-2014, 07:32 AM
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You needed to check your magnetic drain plug anyhow ..
Old 05-21-2014, 07:36 AM
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My 88 does this also. If you get on it making a turn from stopped it feels like the clutch is chattering.
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Old 05-21-2014, 07:43 AM
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One gear oil change isn't likely to rid the tranny of the offending friction modifier. Might take 2 or 3 changes. BTDT with a customer's car.

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Old 05-21-2014, 07:44 AM
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offending friction modifier? This special sauce from kendall has friction modifier. Everyone with LSD says you need friction modifier. Not knowing what kind of fluid my tranny had I went ahead and added some friction modifier. My LSD grinds and crunches the same as before and after I added friction modifier. Now I am going to completely drain all the fluid and refill with kendall 80w90 Limited Slip (which means it has a friction modifier). Can you explain why I would need to change the oil 2-3 times? And by that do you mean buy 12 qts of kendall so that I can fill, drain and trash about 9 quarts?
Old 05-21-2014, 08:36 AM
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"Everyone"? What does Porsche say? They've traditionally specifically discouraged the use of such additives. "Porsche style synchros" (they're used by other manufacturers) operate on friction. Additives which reduce the friction are detrimental to the operation of the synchro rings. It's been a LONG time since I owned a Porsche with an LSD , and maybe things have changed. Or not. As for the flushing, that's just what I meant. The friction modifiers can stay in the system if you only drain and refill once, especially if they're using MoSO2.

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Old 05-21-2014, 10:13 AM
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Quote:
Ok I'll order some Kendall tranny sauce and will report back. This is one of those moments where I'll be shocked if by changing the fluid this metal on metal thrashing sensation is fixed. Guess it's the cheapest possible fix so it's worth a shot.
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A common failure point on the ZF LSD is the bearing flange. It snaps off right at the edge of the diff body. If this were to happen, in tight turns you would be torquing the inner bevel gears out of alignment with the spiders and potentially damaging them.

I'm on a phone right now. I've got an LSD here with this sort of failure that I will photograph and post this afternoon.

I agree with RedCoupe, btw. I only ever suggest friction modifiers for a casually driven street car. For a track vehicle the friction modifiers pose two problems. One is they are too slippery for the synchros. The other is that they make the LSD less effective. They make it lock less.

The absolute worst offender is Redline Shockproof. Just went through this with a Conti race team. They called up complaining that no matter how tight they stacked the LSD or how much preload they introduced they couldn't get the LSD to effectively lock. They wanted me to make them custom super duper ramps for it. I asked what gear oil. They had that Redline garbage in there. Put in some Delvac and problem magically went away.
Old 05-21-2014, 10:18 AM
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Ok so if the bearing flange is broken how would I know? Would tranny fluid leak out? Also if the bearing flange is broken and this leads to more damage...is the damage contained in the LSD or are you saying the actual tranny will be damaged by the broken bearing flange?


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Old 05-21-2014, 10:25 AM
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Remember to loosen the fill plug BEFORE you take out the drain plug!
So that you know you can refill it.
Bob

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Old 05-21-2014, 11:21 AM
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